How Social Security Numbers Work

Social Security created SSNs in 1936. Back then, computers didn't yet exist, and Social Security personnel needed a way to quickly sort the vast numbers of applications that arrived at the agency's office in Baltimore, where the files were organized by region as well as alphabetically. "It was really just a bookkeeping device for our own internal use and was never intended to be anything more than that," a historical article on the Social Security website explains .

To accomplish that, Social Security officials came up with a complex system of nine-digit SSNs, with three different parts that conveyed different pieces of identifying information. The first set of three digits was the Area Number, and originally represented the state in which a person first applied for a Social Security card. The numbers started in the northeast and moved westward, so that northeasterners had the lowest numbers, and people in the West Coast had the highest ones [source: SSA].

The middle two digits, the Group Number, helped break the applicants in a region into smaller blocks. Instead of being assigned consecutively, all the odd numbers from 01 to 99 were assigned, and then all the even numbers from 10 to 98 were used. Once those had been assigned, even groups 02 through 08 were used, and then odd groups 11 through 99 [source: SSA].

Finally, SSNs were given a four-digit ending sequence called the Serial Number, whose numbers were assigned consecutively from 0001 to 9999 [source: SSA].

In 1972, Social Security altered its system slightly. Instead of assigning Area Numbers according to the state where the office to which a person applied was located, Social Security began using the ZIP code of the person's address on the application [source: SSA].

In 2011, Social Security dropped the old system completely, because it limited the number of SSNs available to each state, which eventually might have resulted in Social Security running out of available numbers. Instead, Social Security adopted a new policy called SSN randomization. It eliminated the geographical significance of the initial three digits (that is, the ones that used to be the Area Number). For the first time, it also began using the sequences 000, 666 and 900-999. The significance of the middle two digits, which used to be the Group number, was done away with as well [source: SSA]. The Serial Number was left unchanged.

Social Security says that the change helps give Americans additional protection against identity theft, because it makes it harder for crooks who obtain the last four digits of a person's SSN to use public information to reconstruct the rest of the number [source: Randomization FAQ].